A noise reducing device for achieving high image quality in video apparatuses will be described. Examples of the noise reducing device include a two-dimensional noise reducing device reducing noise based on a signal in the same field or frame and a three-dimensional noise reducing device obtaining a signal difference between fields or frames by use of a memory or the like and reducing noise based on the difference (for example, see Japanese Laid-Open Patent Application No. H03-79168).
First, the operation of a conventional three-dimensional noise reducing device will briefly be described.
FIG. 9 shows an example of the structure of an image forming apparatus having the conventional three-dimensional noise reducing device.
In FIG. 9, reference numeral 101 represents a solid-state image sensing device (hereinafter, referred to as CCD) converting incident light into an electric video signal. Reference numeral 102 represents an automatic gain control amplifier circuit (hereinafter, referred to as AGC circuit) performing amplification so that the output signal level is maintained constant even when the output signal from the CCD 101 is changed. Reference numeral 103 represents an analog-to-digital converter (hereinafter, referred to as A/D converter) converting the analog video signal from the AGC circuit 102 into a digital video signal. Reference numeral 104 represents a subtracter performing a subtraction of one input signal from the other input signal. Reference numeral 105 represents a nonlinear processing circuit performing nonlinear processing on the output of the subtracter 104. Reference numeral 106 represents a subtracter subtracting the output of the nonlinear processing circuit 105 from the output signal of the A/D converter 103 for outputting a video signal SO. Reference numeral 107 represents a field memory outputting the output video signal SO to the subtracter 104 as a reference signal delayed by one field. Reference numeral 108 represents a nonlinear processing control circuit changing the characteristic of the nonlinear processing circuit 105 in accordance with the factor of amplification of the AGC circuit 102.
The operation of the conventional three-dimensional noise reducing device having the above-described structure will be described.
When the electric signal from the CCD 101 is inputted to the AGC circuit 102, in a case where the level of the input signal is not less than a predetermined level, the signal is not amplified and is outputted with the factor of amplification being 1. In a case where the level of the input signal is not more than the predetermined level, the signal is amplified with an increased factor of amplification so that the signal is of the predetermined level.
Then, the output signal of the AGC circuit 102 is converted into a digital video signal by the A/D converter 103.
The video signal converted into a digital signal by the A/D converter 103 is inputted to the subtracter 104, undergoes the subtraction with the output signal of the field memory 107, and is outputted as a difference signal. Since the output signal of the field memory 107 is a signal which is an already-noise-reduced output video signal SO delayed by one field, when the video signal is representative of an object of image formation that is stationary, the video signals of the fields are substantially the same. Therefore, hardly any video signals are included in the difference signal, and only noise components are extracted.
Therefore, by subtracting the output signal of the subtracter 104 from the output signal of the A/D converter 103 by the subtracter 106, a noise-reduced new output video signal SO is outputted. The new output video signal SO is inputted to the field memory 107 as the reference signal, and is used for the noise reduction of the video signal of the next field.
In a moving image where the object of image formation is moving, since the video signals of the fields are different from one another, a large number of video signals are included in the difference signal. Therefore, if the difference signal is subtracted as it is, although noise is reduced in the output video signal SO, the image signal of the immediately preceding field is added at the same time, so that after-image degradation is caused in the image plane.
This defect is handled as follows: Based on a statistical reason that variations with time of the amplitude of noise are generally smaller than those of video signals, regarding a part of the difference signal with a large amplitude as a video signal and a part thereof with a small amplitude as noise, only the part with the small amplitude is extracted by the nonlinear processing circuit 105. With a signal that is the extracted part with the small amplitude as the difference signal, the difference signal is subtracted from the A/D converted video signal by the subtracter 106, whereby a video signal SO where noise has been reduced without any after-image can be outputted.
FIG. 10 is a view showing an example of the characteristic of the nonlinear processing circuit 105. The horizontal axis represents the input of the nonlinear processing circuit 105, and the vertical axis represents the output thereof. “a” and “K” are characteristic parameters characterizing this characteristic.
When the absolute value of the input level of the nonlinear processing circuit that is the level of the output of the subtracter 104 is not more than a, regarding the output of the subtracter 104 as being substantially all a noise signal, a signal having a magnitude of K·x (K<1, x is the input of the nonlinear processing circuit and |x|≦a) and whose level is reduced by a multiplication by the characteristic parameter K is outputted as the difference signal.
When the absolute value of the input signal level of the nonlinear processing circuit is not less than “a” and less than “2a”, the output of the subtracter 104 includes both noise and video signals. Therefore, regarding the ratio of the video signals as increasing as the level increases, the output is reduced with K as the proportionality constant as the absolute value of the input signal level approaches from a to 2a, and a signal of the absolute value of a level close to zero is outputted as the difference signal. When the absolute value of the input level of the nonlinear processing circuit is not less than 2a, regarding the output of the subtracter 104 as being all a video signal, the output to the subtracter 106 is not made.
According to this nonlinear processing characteristic, a signal where the absolute value of the signal level of the input of the nonlinear processing circuit, that is, the output of the subtracter 104 is not more than 2a is regarded as including noise, and a signal reduced by the nonlinear processing in accordance with the signal level is outputted to the subtracter 106 as the difference signal, so that noise can be reduced while after-image degradation is suppressed. The entire disclosure of the above-described Japanese Laid-Open Patent Application No. H03-79168 are incorporated herein by citation (reference) in its entirety.
When the quantity of the light incident on the CCD 101 decreases to reduce the level of the output video signal from the CCD 101, since the noise level is constant irrespective of the output signal level, the S/N ratio deteriorates. When the level is further reduced to a level lower than a predetermined level, the AGC circuit 102 amplifies the input signal so that the predetermined level is maintained. However, since both the signal and noise are amplified with the same factor of amplification with the S/N ratio being degraded, the noise level increases.
On the other hand, the nonlinear processing control circuit 108 changes the characteristic of the nonlinear processing circuit in accordance with the factor of amplification of the AGC circuit 102. Specifically, by setting the value of the characteristic parameter “a” to a high value, the level of the signal regarded as noise is increased. By doing this, even when the noise included in the video signal is increased by the amplification by the AGC circuit 102, it can be reduced.
However, in the output of the subtracter 105, it is impossible to distinguish by the magnitude of the amplitude between the video signal intrinsically having a predetermined amplitude and the noise whose level is increased by the amplification. Therefore, when the value of the characteristic parameter “a” is set to a high value in accordance with the amplified noise level, the video signal which is intrinsically necessary is reduced together with noise by the nonlinear processing circuit 105. Consequently, the output of the nonlinear processing circuit 105 includes a large number of video signals in the field memory 107, so that the after-image degradation in the output video signal SO increases.
That is, when it is necessary to amplify the video signal by the AGC amplifier circuit 102 because the level of the output video signal from the CCD 101 is reduced, since noise is largely amplified compared to the video signal, it is difficult to perform noise removal with consideration of after-image degradation by the nonlinear processing circuit 105 that distinguishes between noise and the video signal based on a statistical reason that variations with time of the amplitude of noise are smaller than those of video signals.